Who would wear pink organza to a mandatory psychiatric evaluation? Villanelle in Killing Eve, played by Jodie Comer.
Photograph: Robert Viglasky/Sid Gentle Films

You can tell a TV villain has made their mark on popular culture if they become a Halloween costume. This week, for every Trumpian wig or handmaid’s bonnet seen at a Halloween party, there was a giant pink dress based on the one worn by the assassin Villanelle, Killing Eve’s antihero.

It has become impossible to talk about the BBC’s eight-part hit – the final instalment of which airs on BBC One on Saturday – without talking about the clothes. Yes, the pink organza dress by Molly Goddard worn with black Balenciaga boots in episode three that subsequently broke the internet, but also what was to come.

Each week we have devoured the fashions. See the Dries Van Noten suit worn by Villanelle to murderous ends in Berlin, or the girlish Paige denim cut-offs worn to shimmy up a drainpipe in Tuscany. And then there is Eve, the brilliant MI5 operative whose ineptness when it comes to getting dressed is partly what attracts the assassin. The green scarf that becomes a prized trophy, the scene-stealing jumper-attached-to-the-shirt and then, of course, the monochrome Roland Mouret one planted in her suitcase, which flipped the power between hunter and hunted. Clothing repels and attracts them, leaving Villanelle torn between trying to hunt her and style her.

Based on the novellas by the journalist Luke Jennings, Killing Eve follows Eve Polastri (Sandra Oh), a semi-happily married desk-based MI5 operative searching for the young assassin played by Jodie Comer. It begins with ice cream, a Lanvin dress and a murder in Vienna, but as the body count goes up, so too do the costume changes.

“Fashion is used as Villanelle dresses carefully for her kills. It’s important to her, part of the ritual,” explains Jennings. Jennings had sent a Pinterest board to the producer and writer, Fleabag’s Phoebe Waller-Bridge, suggesting everything from bridge card rooms to Russian fur hats. It all goes back to the books. In one of the narratives, she murders a mafia boss in “a silk Valentino dress”. In another she wears Vivienne Tam to murder a Chinese hacker. Jennings describes Villanelle’s aesthetic as “goofy couture”, and Eve’s as “defined by her incompetence at shopping”.

Their contrasting aesthetic is key, partly because they define themselves against one another. Villanelle’s obsession with Eve’s “really great hair” was written in after Oh was cast and becomes its own subplot. Comer’s skin, which glows across the Berlin nightclub in perhaps the only moment when the drama gets upstaged by something other than Villanelle’s clothes, has also become a hot topic. Her facialist, Jasmina Vico, explains: “I specifically focused on achieving innocence, a youthful glow”, in part to “underpin Villanelle’s character – to add luminosity.”

Killing Eve captured our collective imaginations for very 2018 reasons. Here was a cat-and-mouse tale which in casting women in those roles subverted standard thriller tropes. Throughout it, women replace men. Eve has a kind, put-upon husband and a male boss who becomes her deputy, while Villanelle has a male handler who she repeatedly outsmarts, an adoring beau, not to mention a largely male body count.

The only person who outwits anyone is MI6 boss Carolyn Martens (Fiona Shaw), whose power becomes apparent in Russia when she wears a fur hat (made from the fur collar of a coat). In an era when the best female characters are either troubled (see all Scandi noir) or repressed (The Handmaid’s Tale) it is refreshing to see women defined by their neuroses. Incidentally, in the novels Martens was a man.

For this reason it’s not reductive to discuss the clothes, explains Jennings, particularly regarding Villanelle. “The clothes are not meant to make her look attractive. She wears what she does because she can do what she wants.” Villanelle grew up as Oksana in Perm, Russia, in relative poverty “where you spend 50 rubles on a pair of knock-off jeans”. Now Villanelle spends all her money on clothes, as her handler Konstantin remarks early on. “They’re her indulgence. It’s not about attracting attention, it’s about feeling glorious and subversive and in control.” If anything, her wardrobe is a sign of financial independence.

By contrast, costume designer Phoebe De Gaye was more focused on using clothes as tools for seduction. A Bafta-award winning costume designer, she is the woman who put Delboy in a shearling coat and flatcap, so understands the way a costume can make its mark in culture. “Villanelle is aware of the effect she’s having, but makes it impossible for us to pin her down. Like a chameleon.” Denim hotpants are “precisely what a girl in her mid-20s might wear in Italy”. On one level, she is hiding in plain sight. On another, “she blends right in”.

For Eve, she was told to “frump her up”. Her tactic was to buy clothes from charity shops – the green scarf with the zebra print, the jumper-attached-to-the-shirt – wet them, scrunch them up and put them in a bag to dry to make it look like it had been left in the corner. She wears a lot of Uniqlo basics, some M&S. The scuffed anorak worn throughout was Oh’s own.

Given Eve’s job, “it’s important that she is forgettable”, explains De Gaye, pointing to the neutrals, greys and beige tones of her wardrobe. “If you don’t remember what she looks like then it’s been a success.”

If Villanelle weaponises her femininity, using it to charm and disarm her victims, then the clothes are integral to that. She uses feminine dresses in obvious ways – see the prairie-style Burberry dress she wears to gain access to someone’s bedroom, before stabbing them in the eye, and the actual “damsel in distress” costume hanging in her bathroom – but other outfits are more baffling. Who would wear a faux-fleece coat in freezing Russia, or pink organza to a mandatory psychiatric evaluation?

It’s hard to remember a thriller in which clothes, both drab and decadent, played such a starring role.

When not being the Observer’s dance critic, Luke Jennings was self-publishing thrillers about two mutually obsessed women. Little did he imagine they would lead him - with Fleabag’s Phoebe Waller-Bridge - to create a subversive TV smash...